In military situations, the eyes of personnel and the detectors of sensitive optical instruments, such as sensors, need protection from damaging laser radiation. The damage can be intentional from offensively used lasers, or inadvertent, as from lasers used for range finders, targeting, and measuring devices. In non-military situations, it is necessary to provide protection from laser radiation to individuals working near or with laser containing equipment. The radiation may be in the ultraviolet, visible, or the infra-red regions of the spectrum.
To be effective these protective filters must have a high optical density at the selected wavelength, a narrow spectral bandwidth, maximum transmission outside the selected band, and excellent optical properties. The rejection maximum of the filter should closely correspond to the wavelength to be rejected. Strong rejection of a narrow, selected portion of the spectrum allows protection from discrete laser lines yet allows for good photopic transmission, or visibility, since the rest of the spectrum passes through the filter.
In many situations it is also necessary for the filter to provide protection against several wavelengths. In practical situations it is frequently necessary for the filter to offer protection from a wide range of incident angles. This is accomplished with proper optical design of the filter and the use of sufficient bandwidth. In general, there is a trade-off between design, angular bandwidth, and photopic transmission.
In addition to the optical requirements, a rugged environmentally stable technology is required. The filter must not change its optical properties under a wide variety of environmental conditions. It is also desirable for the filter to be compatible with a variety of different substrates including different glasses and plastics such as polycarbonate. Polycarbonate is the preferred substrate for military applications because it is light weight and also offers ballistic protection.
Current filter technologies include: absorbing dyes, phosphate glasses, dielectric coatings, and reflective holographic optical elements. Absorbing dyes have several drawbacks: reduced photopic transmission due to broad absorption bands and decrease in absorption due to photobleaching and/or photodegradation. Phosphate glasses are useful for only limited regions of the visible spectrum and, thus, can not be used to prepare filters which provide protection against both visible and infra-red laser radiation. Both dielectric coatings and reflective holographic optical elements offer the advantages of high optical density and narrow band rejection. Dielectric coatings, however, are expensive, difficult to manufacture, and can not readily be attached to complex optical surfaces. Reflective holographic optical elements also offer the additional advantage of complex refractive design. This design, which allows protection from a wide range of incident angles, is not possible with either dielectric coatings or the other types of filters.
Dichromated gelatin is currently the material of choice for the manufacture of reflective holographic optical elements due to high diffraction efficiency and low noise characteristics. However, the material has poor shelf life and requires wet processing. Wet processing may cause the holographic notch filter to change during processing due to the swelling and shrinking of the gelatin during processing thus changing it optical properties and introducing optical aberrations. Thus, it is difficult and time consuming to reproducibly make high quality holographic notch filters with dichromated gelatin. In addition, due to moisture sensitivity of the gelatin, the holographic notch filter must be hermetically sealed against moisture. This is particularly difficult to achieve when polycarbonate or other plastic substances are used due to the moisture porosity of these substrates. Dichromated gelatin is also difficult to coat onto polycarbonate in multilayer configurations which are needed for filters that reject more than one laser line.
Substantially solid, photopolymer films have heretofore been proposed for making holograms. Haugh U.S. Pat. No. 3,658,516, for instance, discloses the preparation of stable, high resolution holograms from solid, photopolymerizable films by a single step process wherein a permanent refractive index image is obtained by a single exposure to a coherent radiation source bearing holographic information. The holographic image thus formed is not destroyed by subsequent uniform exposure to light, but rather is fixed or enhanced.
Despite the many advantages of the materials proposed by Haugh, they offer limited holographic response and application has been limited to transmission holograms where the holographic image is viewed by light transmitted through the imaged material. Moreover, the materials proposed by Haugh have little or no reflection efficiency when imaged to form a reflection hologram.
Thus, a need continues to exist for materials for improved laser protective filters, particularly materials for holographic notch filters. These materials must have excellent optical and holographic properties, must permit ease of manufacture and the production of reproducible holographic mirrors, must be compatible with a variety of substrates including glass and polycarbonate, and must have excellent environmental stability. In addition, they must be capable of being imaged on another substrate and transferred to the final substrate and being imaged directly on the final substrate. They must also be capable of being coated or laminated in multilayer configurations.